Question

Under the topic of aqueous chemical equilibria, outline the quantitative treatment of a single acid in solution. Indicate in your answer, where a mass balance expression is utilised.

Answer 1

Aqueous solutions often contain species that interact with one another and water to yield two or more simultaneous equilibria.

Mass-balance equations relate the equilibrium concentrations of various species in a solution to one another and to the analytical concentrations of the various solutes.

These equations are a direct result of the conservation of mass and moles.

Writing mass balance equations may be as straightforward as the case of a weak acid:

HA+ H₂O ⇔ H₃O⁺ + A^-

2H₂O ⇔ H₃O⁺ + OH^-

The only source of the two A^- containing species, HA and A^- , is the original solute, HA, whose analytical concentration is c_HA:

c_HA = [HA] + [A^- ]

Hydronium ions in the solution come from two sources:

the dissociation of HA and the dissociation of water.

[H₃O⁺] = [H₃O⁺]_HA + [H₃O⁺]_H2O

The concentration of hydronium from the dissociation of the acid [H₃O⁺]_HA is equal to [A^-], and the hydronium concentration from water [H₃O⁺]_H2O is equal to [OH^-].

Thus, [H₃O⁺] = [A^- ] + [OH^- ]

This is referred to as the proton balance equation because it accounts for all sources of protons.